#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
int main()
{
	int a = 0;
	int b = 0;
	scanf("%d% %d", &a, &b);
	printf("%d\n", a * b);
	return 0;
}

int main()
{
	float a = 0;
	float b = 0;
	float h = 0;
	scanf("%f%f%f", &a, &b, &h);
	float c = 0;
	c = ((a + b) * h) / 2;
	printf("%f", c);
	return 0;
}